1. Field of the Invention
The present invention relates to a preparation method of an alkylene carbonate by reacting alkylene oxide and carbon dioxide in the presence of a catalyst system comprising metal halide and pyridine or pyridine, derivative.
2. Description of the Background Art
Alkylene carbonates are used in polycarbonate synthesis, as a solvent for polymer electrolyte, an intermediate in pharmaceutical process, an oxyalkylation agent in dyestuff synthesis, a protectant in processing plant and a solvent in textile production process.
Alkylene carbonate has been prepared by reacting carbon dioxide and alkylene oxide in the presence of a catalyst, represented in Scheme 1. 
wherein, R1 and R2 are each independently H, C1-C4 alkyl or phenyl group.
In the above reaction, however, there is a limitation that alkylene oxide either decomposes or polymerizes at higher reaction temperatures.
Many catalysts have been developed including inorganic salts, phosphonium halide and ammonium halides. For instance, Japanese Laid-Open Patent No S59-13776 introduced a method of using tetraalkyl halide such as tributyl methyl phosphonium iodide as a catalyst. Japanese Laid-Open Patent No. H9-67365 introduced a method of using KI as a catalyst and Japanese Laid-Open Patent No. H9-235252 describes a method of using polystyrene copolymer containing quatemary phosphonium groups.
These patents claim that the product yield is 50-95% when the reaction is performed at 100-170xc2x0 C. for 1-5 hours. However, in order to produce alkylene carbonate in high yield, long reaction time and high reaction temperature are required. Also the water content in the raw materials, carbon dioxide and alkylene oxide has to be reduced to than a few hundred ppms.
Japanese Laid-Open Patent No. H7-206846 introduced a method of using an ion change resin substituted with the catalysts such as CsOH, RbOH and ammonium halides. In U.S. Pat. No. 4,233,221, a method of using DOWEX and Amberlite ion exchange resin was reported with a low yield of 30-80% at 80-100xc2x0 C.
Besides the above-mentioned materials, a phthalocyanine complex containing Co, Cr, Fe, Mn, Ni, Ti, V, or ZR has been used as catalysts. Also in Japanese Laid-Open Patent No. H7-206847, a catalyst system using a heteropolyacid whose hydrogen ion is substituted by Rubidium or Cesium ion was introduced. These two cases, however, require expensive catalysts with low yield of 30-90% at relatively high reaction temperature of 120-180xc2x0 C.
As mentioned above, the catalysts disclosed in the above arts have one or more problems in terms of activity, reaction condition, cost, water sensitivity, etc.
Therefore the object of the present invention is to provide a method of producing alkylene carbonate with a high yield in a short reaction time under mild reaction conditions.
As a result of the efforts made to achieve the above aim, the present inventors have found that a catalyst system comprising pyridines and metal halide compounds is more effective than the conventional catalyst system in producing alkylene carbonates by reacting alkylene oxide and carbon dioxide.
The present invention provide a method to prepare alkylene carbonate of the formula (1) 
wherein, each of R1 and R2 is independently H, C1-C4 alkyl or phenyl group;
which method characterized by reacting alkylene oxide with carbon dioxide in the presence of a catalyst system comprising a) metal halide [MXm] and b) pyridine or pyridine derivative [Py], wherein Py is selected from a group of pyridines; M is a metal atom selected from the group consisting of Zn, Fe, Mn, Pb and In; X is a halogen selected from the group consisting of Cl, Br and I; and m is 2 or 3.
The pyridine derivatives include the compounds having the structures of the formulae (2), (3) or (4), and a copolymer between polyvinyl pyridine (PVP) and polyvinyl pyridine 
wherein R3, R4 and R5 are each independently H, C1-C4 alkyl or phenyl; each of x, y and z is independently an integer from 0 to 3; and c is an integer from 2 to 4.
The molar ratio of pyridine [Py] to metal halide [MXm] is 10:1xcx9c1:5, and more preferably 3:2xcx9c1:1.
The amount of metal halide is preferably 0.005xcx9c0.1 moles per mole of alkylene oxide. In case the amount of the metal halide used is less than 0.005 mol, reaction becomes too slow. And in case the amount of the catalyst is more than 0.1 mol, the reaction rate and selectivity do not improve any further.
Considering the equipment and operating cost, it is preferable to operate a reaction at a pressure of 10xcx9c100 atm.
The reaction temperature is preferably 60xcx9c140xc2x0 C. The reaction proceeds too slow at lower reaction temperature lower than 60xc2x0 C. When the reaction temperature is too high, alkylene oxide either decomposes or undergoes a self-polymerization reaction and the reaction becomes non-specific.
Although the above reaction could be performed in the absence of the solvent, it is possible to use solvent to prevent excess heat production during the reaction. It is preferable to use alkylene carbonate that is produced from the raw material alkylene oxide as a solvent. For instance, ethylene carbonate is a preferable solvent when ethylene carbonate is synthesized from ethylene oxide and propylene carbonate is preferable when propylene carbonate is synthesized from propylene oxide.
The reaction could be performed by a batch process using the reactor provided with a stirrer or by a continuous process using a bubble column. The invention will be further illustrated by the following examples, but not limited to the examples given.